Need Help with Excel problem (VSTO)

wbkSource.Sheets(strFromSht).Copy(wbk.Sheets(1));

Error	2	Non-invocable member 'Microsoft.Office.Interop.Excel._Workbook.Sheets' cannot be used like a method.	 

private String strWorkBookLocation = "C:\\PivotFiles\\source.xls";
       private String strWbkLocation      = "C:\\PivotFiles\\temp.xls";
       private String cstrPivotTableSheet = "Pivot Table";
       private String strFromSht          = "Source";

               Excel.Application app;
               Excel.Workbook wbk;
               Excel.Worksheet sht;
               Excel.Worksheet sh;
               Excel.Workbook wbkSource;
               DataSet dSet = new DataSet();
               DataGrid dg = new DataGrid();
               int y = 2;
               int x = 0;

               try
               {


                   FetchMFRData(ref dSet, "12/24/2010", "12/24/2010");
                   dg.DataSource = dSet;
                   dg.DataBind();
                   FileStream fs = new FileStream(strWorkBookLocation, FileMode.Create);
                   StreamWriter tw = new StreamWriter(fs);
                   HtmlTextWriter writer = new HtmlTextWriter(tw);
                   dg.RenderControl(writer);
                   writer.Flush();
                   writer.Close();
                   tw.Close();
                   fs.Close();

                   app = new Excel.Application();
                   app.DisplayAlerts = false;

                   wbkSource = app.Workbooks.Open(strWorkBookLocation,
                               Type.Missing, Type.Missing, Type.Missing, Type.Missing,
                               Type.Missing, Type.Missing, Type.Missing, Type.Missing,
                               Type.Missing, Type.Missing, Type.Missing, Type.Missing,
                               Type.Missing, Type.Missing);
                   wbk       = app.Workbooks.Open(strWbkLocation,
                               Type.Missing, Type.Missing, Type.Missing, Type.Missing,
                               Type.Missing, Type.Missing, Type.Missing, Type.Missing,
                               Type.Missing, Type.Missing, Type.Missing, Type.Missing,
                               Type.Missing, Type.Missing);

                   // This loop is used for deleting all sheets in the workbook
                   // except the Pivot table sheet.  Generally the only reason
                   // another sheet may be present is if there was an error
                   // during another run and the source sheet didn't get deleted.

                   foreach (Excel.Worksheet sheet in wbk.Sheets)
                   {
                       if (sheet.Name.Equals(cstrPivotTableSheet) == false)
                           sheet.Delete();
                   }

                  wbkSource.Sheets(strFromSht).Copy(wbk.Sheets(1));


               }
               catch (Exception ex)
               {
               }
           }
       }